Torsion bar springs are, of course, quite old in the art and have been used for many purposes including the resilient suspension of vehicles on tracks or wheels. The life span of through hardened torsion bars used on heavy, high speed military or lumber vehicles in off-road environments have not been completely satisfactory in the past. The prior art bars not only creep under severe torsional loads, but also break too frequently, with the break almost always occuring near one end of the bar at the anchor point. Because the breakage occurs at one of the anchoring ends of the bar, one practice previously followed to extend the uniformity and life span of torsion bars was to make the ends larger thus requiring larger and more expensive mounting for the torsion bars.
U.S. Pat. No. 2,843,374 which issued to Boegehold on July 15, 1958 discloses one method of forming torsion bars. In accordance with this prior art method, a high carbon steel shaft of between 0.72 to 0.95% carbon and a 0.30 to 0.60% manganese is required. The high carbon steel bar is austenized by heating in a neutral salt bath at 1500.degree. F. to 1550.degree. F. for 45 minutes. This long heating interval is followed by rapid cooling of the bar as by quenching in a caustic solution. The bar is thereafter tempered at less than 500.degree. F. to provide a case or shell hardness of about 55 to 60 Rockwell C, and a core hardness of about 38 to 47 Rockwell C. In the "as quenched" condition, the shell or case is said to have a hardness of about 62 to 67 Rockwell C for a relatively shallow depth. In fatigue tests, the patentee indicates that failures occurred with a series of bars after 700,000 cycles, and that one bar was run for 1,000,000 cycles without failure under a stress range of 39,000 to 100,000 pounds per square inch.